Move cli complexity into the library

bkl.go

@@ -1,8 +1,8 @@
 // Package bkl implements a layered configuration language parser.
 //
-//  - Language & tool documentation: https://bkl.gopatchy.io/
-//  - Go library source: https://github.com/gopatchy/bkl
-//  - Go library documentation: https://pkg.go.dev/github.com/gopatchy/bkl
+//   - Language & tool documentation: https://bkl.gopatchy.io/
+//   - Go library source: https://github.com/gopatchy/bkl
+//   - Go library documentation: https://pkg.go.dev/github.com/gopatchy/bkl
 package bkl
 
 import (
@@ -17,10 +17,10 @@ import (
 // A Parser reads input documents, merges layers, and generates outputs.
 //
 // Terminology:
-//  - Each Parser can read multiple files
-//  - Each file represents a single layer
-//  - Each file contains one or more documents
-//  - Each document generates one or more outputs
+//   - Each Parser can read multiple files
+//   - Each file represents a single layer
+//   - Each file contains one or more documents
+//   - Each document generates one or more outputs
 type Parser struct {
 	docs  []any
 	debug bool
@@ -265,6 +265,52 @@ func (p *Parser) Output(ext string) ([]byte, error) {
 	return bytes.Join(outs, []byte("---\n")), nil
 }
 
+// OutputToFile encodes all documents in the specified format and writes them
+// to the specified output path.
+//
+// If format is "", it is inferred from path's file extension.
+func (p *Parser) OutputToFile(path, format string) error {
+	if format == "" {
+		format = Ext(path)
+	}
+
+	fh, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
+	if err != nil {
+		return fmt.Errorf("%s: %w (%w)", path, err, ErrOutputFile)
+	}
+
+	defer fh.Close()
+
+	err = p.OutputToWriter(fh, format)
+	if err != nil {
+		return fmt.Errorf("%s: %s", path, err)
+	}
+
+	return nil
+}
+
+// OutputToWriter encodes all documents in the specified format and writes them
+// to the specified [io.Writer].
+//
+// If format is "", it defaults to "json-pretty".
+func (p *Parser) OutputToWriter(fh io.Writer, format string) error {
+	if format == "" {
+		format = "json-pretty"
+	}
+
+	out, err := p.Output(format)
+	if err != nil {
+		return err
+	}
+
+	_, err = fh.Write(out)
+	if err != nil {
+		return fmt.Errorf("%w (%w)", err, ErrOutputFile)
+	}
+
+	return nil
+}
+
 func (p *Parser) log(format string, v ...any) {
 	if !p.debug {
 		return

cmd/bkl/main.go

@@ -46,36 +46,20 @@ func main() {
 		}
 	}
 
-	format := "json-pretty"
-
-	if opts.OutputPath != nil {
-		format = bkl.Ext(*opts.OutputPath)
-	}
-
+	format := ""
 	if opts.OutputFormat != nil {
 		format = *opts.OutputFormat
 	}
 
-	out, err := p.Output(format)
+	if opts.OutputPath == nil {
+		err = p.OutputToWriter(os.Stdout, format)
+	} else {
+		err = p.OutputToFile(*opts.OutputPath, format)
+	}
+
 	if err != nil {
 		fatal("%s", err)
 	}
-
-	fh := os.Stdout
-
-	if opts.OutputPath != nil {
-		fh, err = os.OpenFile(*opts.OutputPath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
-		if err != nil {
-			fatal("%s: %s", *opts.OutputPath, err)
-		}
-
-		defer fh.Close()
-	}
-
-	_, err = fh.Write(out)
-	if err != nil {
-		fatal("output: %s", err)
-	}
 }
 
 func fatal(format string, v ...any) {

error.go

@@ -7,8 +7,9 @@ var (
 	Err = fmt.Errorf("bkl error")
 
 	// Format and system errors
-	ErrEncode          = fmt.Errorf("encoding error (%w)", Err)
 	ErrDecode          = fmt.Errorf("decoding error (%w)", Err)
+	ErrEncode          = fmt.Errorf("encoding error (%w)", Err)
+	ErrOutputFile      = fmt.Errorf("error opening output file (%w)", Err)
 	ErrInvalidIndex    = fmt.Errorf("invalid index (%w)", Err)
 	ErrInvalidFilename = fmt.Errorf("invalid filename (%w)", Err)
 	ErrInvalidType     = fmt.Errorf("invalid type (%w)", Err)